Theres only one talent that gives the assassin a bonus if he wears medium armor, using 2 points in this talent will give the assassin 10 crit chance if he has all 5 pieces of gear equipped.
This is a tiny bonus, and is hardly noticeable as time goes on.
On the other hand an assassin can use heavy armor, receive better defensive bonuses and have 2 talent points spare to use more effectively.
Now unless I'm missing something that may have been added defensive wise (for example medium armor is better for taking magic damage and heavy is better for taking melee damage) there is no good reason for an assassin to use medium armor.
My suggestion would be to boost this talent so it's more of a "mandatory choice" in any good assassin build or make some changes to all the armor attributes, making damage types more diverse across different types of armor.
The boost to the talent could be something like "2% on 1 point used + a 5% chance to receive 100% accuracy on your next attack and 4% on 2 points used + a 10% chance to receive 100% accuracy on your next attack" per medium armor piece.
